The present invention relates to an antiskid braking apparatus for a vehicle for preventing locking of wheels of a vehicle at the time of, for example, rapid braking.
FIGS. 6A and 6B are structural views which illustrate a usual antiskid braking apparatus for use in an automobile or a motorcycle. The antiskid braking apparatus 100 comprises a master cylinder 1, a braking mechanism 2 for braking the rotations of wheels, a direction control valve 3, a pressure reducing valve 4, a pump unit 5, a control means 6 and a wheel-speed sensor 7 for detecting the rotational speed of the wheels.
When braking force is applied to the master cylinder 1 from a brake pedal or a brake lever, an internal piston 1a disposed in the master cylinder 1 is pushed in as shown in FIG. 6B so that hydraulic oil (brake fluid) enclosed in the master cylinder 1 is discharged. On the other hand, the braking mechanism 2 comprises a disc rotor 2a so formed as to be integrally rotated with the wheel and a brake caliper 2b holding the disc rotor 2a.
The master cylinder 1 is connected to the direction control valve 3 via a fluid passage 8, the direction control valve 3 being connected to the brake caliper 2b via a fluid passage 9. The fluid passage 8 and the fluid passage 9 are connected to each other by a fluid passage 11. A check valve 12 for permitting only a flow of the hydraulic oil that returns from the brake caliper 2b to the master cylinder 1 is connected to an intermediate position in the fluid passage 11.
The pressure reducing valve 4 is connected to the direction control valve 3 via a fluid passage 13. The fluid passage 13 and the fluid passage 8 are connected to each other through a fluid passage 14. The pump unit 5 includes a pump 5a connected to an intermediate position of the fluid passage 14. Check valves 15 and 16 for permitting only flow of the hydraulic oil that returns from the pressure reducing cylinder 4 to the master cylinder 1 are connected to the upstream and down stream sides of the pump 5a with respect to the flow of the fluid. The pump unit 5 is also equipped with a motor 5b for operating the pump 5a.
The control means 6 is wired to a solenoid 3a for operating the direction control valve 3, the motor 5b and the wheel-speed sensor 7.
When the brake pedal or the brake lever of a vehicle is operated in the state shown in FIG. 6A, the hydraulic oil in the master cylinder 1 is, under pressure, fed to the brake caliper 2b via the fluid passage 8 and the fluid passable 9. The hydraulic pressure causes the brake caliper 2b to hold the disc rotor 2a so as to brake the rotations of the wheels.
When the brake pedal or the brake lever is released, the hydraulic oil fed to the brake caliper 2b returns through the fluid passage 9 and the fluid passage 8 to be again charged into the master cylinder 1 so that the hydraulic pressure is applied to the brake caliper 2b, thus releasing the braking of the wheels.
At the time of rapid braking or braking on a road on which slippage easily takes place, the possibility of locking of the wheels is raised in proportion to the braking force. In such a case, the control means 6 detects a sign of locking of the wheels in response to an input from the foregoing wheel-speed sensor 7 and, if the sign is detected, operates the solenoid 3a to operate the direction control valve 3 from a state shown in FIG. 6A to a state shown in FIG. 6B.
As a result, the fluid passage 8 is closed so that the hydraulic oil in the master cylinder 1 is not sent to the brake caliper 2b. Further, the fluid passage 9 and the fluid passage 13 are communicated with each other so that the hydraulic pressure applied to the brake caliper 2b is relieved to the pressure reducing cylinder 4. As a result, the hydraulic oil is accumulated in the pressure reducing cylinder 4, and then, the force of the brake caliper 2b to hold the disc rotor 2a is relaxed and locking of the wheels is prevented. The hydraulic oil accumulated in the pressure reducing cylinder 4 is returned to the master cylinder 1 by the operation performed by the pump unit 5 so that the piston 1a of the master cylinder 1 is pushed back.
If the sign of locking of the wheels has disappeared, the control means 6 operates to immediately return the direction control valve 3 from the state shown in FIG. 6B to the state shown in FIG. 6A. Therefore, the hydraulic oil discharged from the master cylinder 1 is again fed to the brake caliper 2b so that the brake caliper 2b brakes the rotations of the wheels.
The foregoing operation is quickly repeated (called antiskid operation) so that strong braking force can be obtained while preventing locking of the wheels. As a result, the vehicle can safely be stopped in the shortest distance.
In the antiskid braking apparatus 100 having a structure as described above, if the pump unit 5 is broken down and the hydraulic oil accumulated in the pressure reducing cylinder 4 cannot therefore be returned to the master cylinder 1, it will be considered to employ a case in which the control means 6 detects the failure to continue raising/lowering of the hydraulic pressure applied to the brake caliper 2b until the hydraulic oil in the master cylinder 1 is completely fed to the pressure reducing cylinder 4 or to immediately stop the operation and perform an operation as performed by a usual braking apparatus.
If the raising/lowering of the hydraulic pressure applied to the brake caliper 2b is continued until the hydraulic oil in the master cylinder 1 is completely fed to the pressure reducing cylinder 4 as described above, there are advantages that the antiskid effect can be obtained for a while if the pump unit 5 has been broken down and a driver of a vehicle is able to instantaneously recognize the occurrence of the failure because the stroke of the brake pedal or the brake lever is rapidly lengthened.
However, the fact that the conventional structure has such an arrangement as that the capacity of the pressure reducing cylinder 4 is made larger than the hydraulic pressure discharge capacity which is realized by the full stroke of the master cylinder 1 causes the piston 1a of the master cylinder 1 to reach the bottom and, hence, no braking force is exhibited from this state if the raising/lowering of the hydraulic pressure to be applied to the brake caliper 2b is continued until the hydraulic oil in the master cylinder 1 is completely fed to the pressure reducing cylinder 4. In order to again obtain the braking force in the foregoing case, the brake pedal must be again depressed or the brake pedal must be again squeezed, thus arising a risk that the braking distance is lengthened.
On the other hand, if the antiskid operation is immediately stopped after the pump unit 5 has been broken down and the operation is performed in a manner similar to that of a usual braking apparatus as described above, the antiskid operation is rapidly stopped during the braking operation with which the antiskid operation is performed. In this case, there is a risk that the wheels are locked too rapidly for the driver of the vehicle to take any countermeasure.
Furthermore, in the antiskid braking apparatus 100, if the operation of the pump unit 5 causes the pump pressure from the pump unit 5 and the manual braking operation force to be simultaneously applied to the master cylinder 1, the pressure in the master cylinder 1 is caused to be raised excessively. As a result, there is a risk that, for example, an oil seal disposed between the master cylinder 1 and the piston 1a is damaged.
Further, the pump pressure from the pump unit 5 intermittently and strongly pushes back the piston 1a of the master cylinder 1, which may lead to a fact that vibration-like kick back is applied on the brake pedal or the brake lever, causing disagreeable operation touch to be created.